The recent trend is to make automatic transmissions which are more stepped or graduated to provide slightly more forward gear ratios that can be selected so that shifting does not occur in too large steps, and therefore provide a smoother gearshift, as a way to realize improved fuel-efficiency and drivability. In order to realize this graduation of selectable gear ratios, gearshift assemblies have been designed for automatic transmissions which are formed by combining a reduction planetary gearset located in a front portion of the transmission nearest an engine which reduces rotation speed from the engine, and, a shift mechanism located in a rear portion of the transmission to receive the reduced rotation from the reduction planetary gearset. The reduction planetary gearset and the shift mechanism are separated by a midway wall which is disposed inside the transmission case perpendicularly to the axis of the transmission. Such a gearshift assembly also includes a direct clutch which transmits rotation from the engine directly to the shift mechanism, and allows several gear ratios to be selectable through a combination of engagement and disengagement of the direct clutch as well as engagement and disengagement of two clutches and brakes contained in the shift mechanism.
It is normal for supply of fluid pressure to a transmission friction element such as a clutch which exists adjacent to the midway wall to be delivered via radial passages formed in the midway wall. This simplifies the structuring of fluid passages to the friction element from a control valve body which is normally on the exterior of the transmission.
Japanese Patent Provisional Publication 2000-220704 discloses a typical hydraulic passage structure. This structure will now be discussed briefly with reference to FIG. 7. A clutch drum d is supported to be freely rotatable on a center boss portion f. Center boss portion f is formed as a single integral body with a midway wall b, and is disposed about an inner circumference of midway wall b to protrudingly extend in the axial direction, such that the inner circumference of midway wall b continues smoothly with an axial-direction hole n inside center boss portion f. Fluid pressure from a radial-direction oil passage m in midway wall b passes through and in order of an axial-direction hole n, a radial-direction hole o, a circumference groove g, and a drum hole k to reach a piston chamber j, to effect engagement of a clutch c. Seal rings h and i are disposed where clutch drum d fits over center boss portion f, to prevent leaking of fluid from both sides of circumference groove g.